Foam sheets using petroleum resin such as polyvinyl chloride (PVC), etc. are widely used in buildings such as houses, mansions, apartments, offices stores, etc.
The foam sheet is manufactured by extrusion or calendaring methods, etc. using resins such as polyvinyl chloride (PVC), etc. But, since entire quantities are obtained from raw materials, which are limited resources such as oil, etc., problems of difficulties of meeting supply and demand, etc. occurring in the future with regards to exhaustion of oil resources, etc. is expected.
Also, even considering the current rising attention to environmental problems, polyvinyl chloride (PVC) foam sheets have problems of easily emitting toxic substances and imposing burden on the environment even when disposed.
In respect to these problems, polylactic acid extracted from plant resources and synthesized is receiving attention to substitute the petroleum resin. Polylactic acid is a resin produced by polymerizing lactic acid, which is obtained from fermenting starch extracted from renewable plant resources (corn, potato, sweet potato, etc.), and is an environmental-friendly resin not only able to reduce CO2 but also able to reduce non-regenerative energy. Korean Patent Laid-open Publication No. 10-2008-0067424 and numerous prior publications mention foam sheets using this polylactic acid.
But, this polylactic acid is easily hydrolyzed in fixed humidity and temperature conditions, and compared to foam sheets produced from prior PVC resin, foam sheets produced from polylactic acid sticks to processing equipments when processing thermal lamination, or lacks viscoelasticity when processing in high temperatures, and thus have disadvantages of process of lamination to multiple layers being not easy. Therefore, improving waterproofing and workability has been a very important task.
Also, polylactic acid is a resin having strong crystallizability and relatively low molecular weight, and since melt strength is low, not only is foaming ratio low during foaming but also there are problems of ununiform open foam cell forming, and thus producing foam sheets with a uniform and closed foam cell structure was required.